The Water-Alternating-Gas (WAG) method where water is combined with gas is a well-known Enhanced Oil Recovery (EOR) method. According to the WAG method, water and gas are injected in succession for as long as petroleum fluids are being produced economically. The purpose of the water slugs is to reduce the mobility of the gas and to widen the swept zone. Many improvements have been proposed for this technique, for example in one method surfactants can be added to the water in order to decrease the water-oil interfacial tension and in another method a foaming agent is added to the water, when foam is created in the presence of gas the mobility of the gas is significantly reduced. The latter method is described in U.S. Pat. No. 3,893,511 or Russian Federation Patent No. 2,039,226. However such a technique requires sophisticated equipment as the parameters of injection must be accurately fulfilled which is difficult to achieve under normal oil field conditions.
U.S. Pat. No. 6,546,962 as well as Russian Federation Patent No. 2,046,931 describe inventions wherein an ejector is used to introduce air or gas into injection water for enhancement of oil recovery. The injection water is supplied to the ejector at a predetermined pressure while at the same time air or associated gas is also being supplied to the ejector. The pressure and velocity of the water passing through the ejector is arranged so as to draw air or gas into the water stream. The amount of gas drawn into the water is preferably capable of being dissolved entirely at the wellhead (or formation) pressure as well as being sufficient to achieve the desired effect in the formation. The ejector uses the energy of the injector pump to accelerate the injection water, thereby reducing the pressure in order to draw in the gas. However, the gas-water mixture stability is low because the size of the gas bubbles cannot be controlled in accordance with these inventions. In addition, a gas/water mixture requires additional compression relative to injection pressure as compared to a water-only system, thus extra energy is needed. The present invention was developed to overcome these and other drawbacks of prior methods and devices by providing an improved method and apparatus for enhanced oil recovery by injection of a gas-liquid mixture into the oil-bearing formation.